1. Field of the Invention
The present invention is related to minimally invasive devices and methods for treatment of biological tissue. More particularly, the invention relates to surgical procedures mediated by diode laser induced modulated plasma in order to achieve specific effects on tissues.
2. Invention Disclosure Statement
Since laser technology was introduced in markets for medical procedures, numerous laser devices have been proposed for tissue removal. Laser energy can be used taking advantage of its different advantageous features. As a consequence, tissue can be ablated, vaporized, liquefied, coagulated, etc, through laser radiation by means of using different treatment parameters, such as power density on the tissue target, energy dose, and by selecting the specific wavelength, etc.
Laser energy can also lead to plasma formation on the matter. Plasma formation is achieved through fast matter ionization with optical breakdown, which is a non linear effect produced when laser radiation is strongly absorbed by irradiated matter and/or with high power density on the target.
Plasma formation is a very complex phenomenon; if the electromagnetic radiation impacts with the target and is absorbed strongly, if there is enough power density, plasma formation is triggered. An optical breakdown starts and an ionization of the matter rapidly expands, generating shockwave and a cavitation effect with crackling noise, if the target medium is water hot vapor bubbles also form. [M. H. Niemz, “Laser Tissue Interaction”: Springer-Verlag ed. 2002—W. Cecchetti, Urology 63 (3), 2004]. A cavitation process with the vapour bubbles give further contribute to shockwave growth. As a consequence, by focusing laser energy on a target material such as a gas, liquid or solid, the latter may be damaged by the sequence of plasma formation with optical breakdown and shockwave generation.
If the target has a high absorption of the laser radiation, the plasma formation can occur even with low levels of laser power and also in continuous emission. Plasma and cavitation phenomena are both associated with strong photo-thermo-ablative and photo-mechanical effects. Inside plasma bubbles, high temperatures of over a thousand degrees arise. The presence of cavitation effects is always associated with shock waves with the typical crackling noise. One example of how this can be achieved is by providing an initial pulse as for instance in the FREDDY device (in this case generated by a flash lamp pumped frequency doubled pulsed Nd-YAG laser). The plasma produced by FREDDY laser, that due to the short duration of pulses, has high photomechanical effect and insignificant thermo ablative effect.
According to the afore-mentioned mechanism of action, laser energy can be applied in two different ways to achieve tissue removal (by means of plasma formation). Indirectly, by focusing it upon a target placed between laser beam and tissue, or directly on target tissue in order to achieve its removal. In the first case, laser energy is transmitted through an optical fiber immersed in water, generating plasma bubbles on fiber tip that produce the damage of the entire tissue that is located in the expansion area of the plasma bubbles.
In U.S. Pat. No. 5,224,942, Beuchat et al. disclose a method and apparatus using laser energy for destroying body tissue which includes a handpiece comprising a surgical tip assembly which is driven by means of laser to achieve optical breakdown, plasma formation and shockwave generation to emulsify or destroy body tissue. As laser is focused on a target (placed inside the handpiece) which vibrates due to plasma formation, mild energy is applied to tissue, which is only emulsified by mechanical vibration of handpiece tip. As a consequence, versatility of this system is limited as it is aimed at treating soft tissue.
U.S. Pat. No. 5,324,282 by Dodick et al., teaches a system based on similar principles. Pulsed laser energy is discharged to strike a metal target, which acts as a transducer converting the electromagnetic energy into shockwaves that are directed to the tissue to be treated. The mechanical shockwaves cause the tissue to fracture.
In U.S. Patent Publication No. 2004/0167504 Thyzel et al. disclose a surgical needle for fracturing tissue comprising a distal operating port which holds tissue. Pulsed laser energy is applied to a target through an optical fiber, generating shockwaves due to plasma formation from the optical breakdown of target, impinging on the tissue to be fractured. This patent is mainly focused on fracturing tissue, so here again system versatility is limited.
Afore-mentioned patents are founded on plasma formation upon a target material, which converts optical breakdown into mechanical vibrations. As a consequence, energy loss occurs in this transduction, diminishing treatment efficacy. Furthermore, mechanical vibrations are not selective with the tissue to be treated, so effects on other tissue rather than tissue to be treated may appear. In other words, not only desired tissue may be affected by vibrations.
When laser radiation is directly focused on tissue in order to achieve its removal, target for radiation is now tissue itself Usually, tissue to be removed is surrounded by liquid and illuminated with laser radiation above a threshold intensity level, it generates plasma bubbles with very high internal temperatures. Thus, tissue undergoes rapid thermal ablation, associated with the mechanical damage of the cavitation effect (example Ho-YAG laser). This method is widely used in order to break calculi, bones, and calcified tissue within the body. This way stones have been fractured by the shockwaves created due to the collapse of bubbles initiated by plasma formation at the tip of fiber optics delivering laser pulses from flash lamp pumped, frequency doubled YAG lasers (FREDDY).
For instance, plasma has been used in medical treatments in the form of ionized Argon gas for the ablation of mucosal layers; TURis resectoscope of Olimpus uses a plasma bubble, generated by electrical device, for the ablation of prostatic tissue for the BPH treatment.
In U.S. Pat. No. 5,071,422, Watson et al. disclose a method for breaking down material within the body, based on a pulsed dye laser source. Optical fiber is inserted in the area to be treated, which is surrounded with liquid and then radiated with pulsed dye laser energy in order to achieve fragmentation by means of shockwaves. This invention basically discloses calculi and stone fragmentation. But if dye laser radiation is not absorbed by stones, plasma formation will not occur and laser lithotripsy will not be effective. The plasma produced by a dye laser produces mainly photomechanical effects. Furthermore, as a pulsed dye laser source is used, frequent maintenance may be required as this source is not a solid-state laser.
U.S. Pat. No. 5,963,575 by Müller et al., discloses a Q-switched laser system for laser lithotripsy. The system incorporates longer pulse duration, increasing plasma formation and consequently shockwave production. Laser source is preferably a Nd:YAG laser, which is a ionic crystal source. As a consequence, it has low efficiency, large dimensions, and needs liquid cooling. Moreover, it requires alignment, as laser radiation is conveyed to the treatment zone by means of minors instead of optical fibers. Furthermore, this technology lacks precision compared to other laser technologies.
In U.S. Pat. No. 4,960,108, Reichel et al. teach a laser-induced lithotripter in which pulsed laser radiation in the vicinity of infrared region is concentrated at a concrement to be destroyed which is surrounded with an aqueous rinsing liquid. Concrement is destroyed by breakdown (plasma) of rinsing liquid, giving rise to shockwave and cavitation. Rinsing liquid includes a metal compound which lowers the energy required for said breakdown.
All previous-mentioned patents only disclose use of laser sources that may be usually voluminous, inaccurate, inefficient and/or requiring frequent maintenance.
Due to the disadvantages and lack of versatility of current plasma formation techniques, a need exists for a device that provides a fast and safe alternative to address their shortcomings.